The present disclosure relates to lithographic methods, and particularly to a method of aligning a lithographic mask to a substrate including a patterned underlying level structure printed with a plurality of lithographic masks and reticles employed for the method.
A reticle, or a lithographic mask, comprises a transparent reticle substrate and a patterned optically opaque coating thereupon; alternatively, a photomask may comprise a transparent reticle substrate with a partially transmissive layer, or with features etched out of the substrate to achieve optical phase shifting by changing the optical path length. The reticle is mounted into an exposure tool, which may be integrated into a tool called a stepper, so that radiation from a source of the exposure tool passes through the reticle and impinges on a photoresist applied to a top surface of a semiconductor substrate. The pattern of the reticle is thus transferred into the photoresist during the exposure so that the photoresist may have the same pattern as the pattern of the reticle after development. The reticle may be repeatedly employed to replicate the pattern in the reticle in the photoresist material on multiple semiconductor substrates. The coating on the reticle is optically opaque at the wavelength of the radiation source. Typical wavelengths of radiation that are employed for photolithography include 365 nm, 248 nm, 193 nm, 157 nm, etc. Such deep ultraviolet (DUV) wavelengths may be employed to pattern features having dimensions of 50 nm or greater in the photoresist.
A pattern in a single level can be formed by multiple lithographic masks. The need to employ multiple lithographic masks to print a pattern in a single level arises because of the limitations on lithographic capabilities. For example, patterns in proximity to each other cannot be printed with fidelity due to optical interferences during exposure. Thus, a single lithographic pattern including minimum size features can be divided into two complementary patterns or a set of more than two patterns the sum of which constitutes a complete pattern. Such division of a complete pattern for a single level into multiple sub-patterns can be performed, for example, at gate level, at contact level, at via levels, and/or at metal line levels.
When multiple exposures are employed to pattern a structure at a single level, alignment of the next level structure to the previous level becomes challenging. The throughput of an exposure tool decreases because the reticle for the new level needs to be aligned to each set of alignment marks associated with a previous level lithographic mask and physically printed on a substrate including a photoresist. The alignment tool then optimizes overlay errors so that the alignment of the image for the current level to each previous level pattern does not exceed a preset limit, or is otherwise optimized to enhance overall alignment between the current level and the previous level. Thus, the total time that the exposure tool spends for alignment increases with the total number of reticles employed to print the previous level, thereby adversely affecting the throughput of the exposure tool.
In addition, an alignment structure pattern is present in a reticle for each printed alignment structure that is lithographically formed on a substrate. Thus, each alignment structure pattern takes up space in a reticle, and reduces area available for printing device structures within a die area.